2D van der Waals materials have rich physical and chemical properties that attract broad research interests. Semiconducting transition metal dichalcogenides (sTMDs) draw special attention among them owing to their unique valley structure and direct band gap transition in the monolayer limit. With the method of substitutional doping/alloying, we can further manually modify their optical, electrical, and magnetic behaviors for next-generation optoelectronics and valleytronics. In this report, I will present my studies on modifying the properties of sTMD monolayers through substitutional doping/alloying. Specifically, the control over dopant concentration and spatial distribution in monolayer sTMDs is first achieved by a liquid-phase precursor-assisted chemical vapor deposition (CVD) method we recently developed. The variation in optical properties due to dopant distribution inhomogeneity is characterized, and a growth mechanism for domains with different dopant concentrations is proposed based on density functional theory (DFT) calculations. Next, I will focus on the novel room-temperature ferromagnetism in monolayer sTMDs introduced by vanadium doping. The temperature dependence of saturation magnetization and coercivity field is studied, and a special thermally induced spin flip (TISF) phenomenon is demonstrated in vanadium-doped WSe2 monolayers. Finally, I will exhibit our efforts on direct observation of magnetic domains in these 2D diluted magnetic semiconductor (DMS) systems, where a field-dependent reversible contrast is reported in vanadium-doped WS2 monolayers from Lorentz transmission electron microscopy (Lorentz-TEM). The signal strength and range are dependent on charge transfer between the substrate, as well as dopant concentration.